dirty pads -- how?

I have a set of Shimano M575 (low end hydro brakes). Despite my efforts to keep the brakes clean, I'm only able to go for about 3 hours before a loose almost all braking on the rear brake. I never touch the rotor and I have minimal chain lube on. I don't know what's causing it but taking the pads off every other ride to bake them is getting old. the pads are resin. is there anything I can do? I don't know if it matters but it almost never rains where I live. I've also had no trouble with the front brakes.